The invention relates to an electric lamp comprising
a lamp vessel of glass having an SiO.sub.2 content of at least 95% by weight, within which an electric element is arranged.
Metal foils each having two major surfaces limited by a first and a second end of the foil and by side edges extending from the first to the second end are connected at their first ends to a respective inner current conductor, which extends to the electric element, and are connected at their second ends to a respective outer current conductor.
The metal foils are surrounded between their first and second ends in a vacuum-tight manner by glass of the lamp vessel;
The major surfaces of each of the metal foils adjoin each other at the side edges at an acute angle, a major surface having a convex curvature transverse to the side edges.
Such a lamp is known from U.S. Pat. No. 3,571,899 (G.E. 3-23-1971).
For lead-through members in lamp vessels of glass having an SiO.sub.2 content of at least 95% by weight, use has to be made of metals having a high melting point, such as, for example, molybdenum and tungsten because of the high processing temperature of this glass. These metals, like bodies consisting mainly of one of these metals, have a thermal coefficient of expansion which is much higher than that of the glass. A vacuum-tight connection of the glass to these metals can nevertheless be obtained provided that the metal is in the form of a foil which, has a small thickness and is knife-shaped (feathered) at its side edges.
The aforementioned U.S. Pat. No. 3,571,899 states that the angle (.alpha.) at which the two major surfaces of the metal foil adjoin each other at the side edges may be at most approximately 5.degree.. Furthermore, according to this Patent Specification, the width of the metal foil (i.e. the distance between the side edges) must be at least approximately a hundred times the largest thickness (i.e. the largest distance between the two major surfaces). This ratio has for a long time been deemed necessary; cf. British Patent Specification No. 551,136 (Siemens Electric Lamps and Supplies Ltd., Feb. 9, 1943), which states a ratio of at least 100 and preferably of approximately 200.
The largest thickness of a metal foil generally lies between 10 and 120 .mu.m and mostly between approximately 30 .mu.m and approximately 100 .mu.m. These thicknesses result in that the width of the foil generally has to be at least approximately 3 mm to approximately 10 mm. Mostly the space available limits in an electric lamp the width of the metal foil, as a result of which, in view of the requirements with respect to the angle (.alpha.) and the ratio between width and thickness, the largest thickness of the foil is determined. Especially in the case where the space available is small and therefore the foil is narrow and consequently very thin, the current density in the foil may be very high because the surface area of the cross-section of the foil is then small. A comparatively large amount of energy is then dissipated in the foil, which gives rise in situ to an unfavorably high temperature and to loss of efficiency of the lamp.
Another consequence of the use of a thin metal foil is that, when a welding connection is made between the metal foil and the inner current conductor or the outer current conductor, rupture is liable to occur due to the fact that the foil melts. Also when the foil is embedded in the glass of the lamp vessel, this operation being effected usually by means of pinch blocks, rupture is liable to occur, for example, due to the fact that the thin foil has become brittle at a welding area. The risk of rupture during welding is reduced according to the aforementioned U.S. Pat. Specification in that during the manufacture of the metal foil there is applied at the area at which a welding connection is to be made a coating which protects the foil at this area from etching liquid used to give the foil convexly curved major surfaces and sharp side edges. However, the total of the manufacturing process as measured in time and money is increased by the additional operations of applying the coating and removing it afer etching.
A thin metal foil also renders it difficult to manipulate this foil, and a unit comprising this foil during the assembly of a lamp, due to the fact that this unit is very weak.